FRIDAY, March 25, 2016 (HealthDay News) — Men who know they've probably been infected with the mosquito-borne Zika virus should not have sex without a condom for six months, according to new federal health guidelines released Friday.

Numerous cases of sexually transmitted Zika infection—which is thought to cause severe birth defects in some cases—have been confirmed in the United States, said officials at the U.S. Centers for Disease Control and Prevention.

"Mounting evidence supports a link between Zika and microcephaly, and possibly other problems such as miscarriage," Dr. Denise Jamieson, co-lead of the Pregnancy and Birth Defects Team of the CDC's Zika Virus Response Team, said during an afternoon news conference.

"The rate of these conditions is not known yet," she said. "We know there is a risk, but it is important to remember that even in places with active Zika transmission women are delivering apparently healthy infants."

The goal of the latest CDC guidelines is to give doctors the best advice possible to share with their patients about pregnancy planning and sex, Jamieson added. However, they are are based on the best evidence to date, and not on a definitive understanding of Zika, she noted.

Zika is a mosquito-borne virus that's been tied to thousands of cases—mainly in Brazil—of a severe birth defect called microcephaly. In microcephaly, a newborn's head is smaller than normal, with the potential for long-term neurological damage.

While the bulk of Zika cases leading to microcephaly may occur via maternal infection during pregnancy, cases of sexual transmission from a man to his female partner have come to light, the CDC said.

A team led by CDC investigator Alexandra Oster notes that, as of March 18, there are now "six confirmed cases of sexual transmission in the United States associated with this outbreak."

Just how long might the Zika virus linger in semen? According to the report, semen collected from one man still showed signs of the virus 62 days after he began to exhibit fever linked to Zika infection.

Zika infection is usually a transient, mild illness in adults, and many cases may occur without symptoms, experts say. However, because of the risk to babies, the CDC is advising that men with known or suspected infection with Zika refrain from sex—or only have sex with a condom—for six months after a diagnosis.

The agency also advises that, for couples involving a man who has traveled to or resides in an area endemic for Zika:

• the couple refrain from sex, or use condoms during sex, throughout the duration of a pregnancy.

• they refrain from sex, or use condoms during sex, for eight weeks if the man has returned from travel to a Zika-endemic area but has not shown signs of infection.

• for couples living in a Zika-endemic area, they refrain from sex or engage in sex only with a condom for as long as active Zika transmission persists in that area.

The latest guidelines also recommend that women who know they've been infected, have no symptoms but have recently been to a Zika-endemic area, or think they might have been exposed via sex, should wait at least eight weeks before trying to get pregnant.

The CDC has also advised that all pregnant women consider postponing travel to any area where Zika virus transmission is ongoing. If a pregnant woman must travel to or live in one of these areas, she should talk to her health-care provider first and strictly follow steps to prevent mosquito bites.

On Friday, CDC officials also said that 273 U.S. residents in 35 states have now tested positive for infection with the Zika virus.

"All are travel-related or sexually transmitted cases," Jamieson said. "In addition, there have been 261 cases reported from Puerto Rico, 14 cases from American Samoa and 11 cases from the U.S. Virgin Islands. Of these, 99 percent are presumed to be locally transmitted by mosquitoes in the territories."

In the majority of Zika infections, symptoms included rash (97 percent of cases), fever and joint pain.

"Zika virus disease should be considered in patients with acute onset of fever, rash, arthralgia [joint pain], or conjunctivitis [pink eye] who traveled to areas with ongoing Zika virus transmission or who had unprotected sex with someone who traveled to one of those areas and developed compatible symptoms within two weeks of returning," the CDC said.

And earlier this month, scientists reported more evidence supporting a link between the Zika virus and microcephaly.

Researchers now believe that one in every 100 pregnant women infected with the virus during the first trimester will give birth to a baby with the birth defect.

The Zika virus is suspected of causing an epidemic that started last spring in Brazil, where there have been more than 5,600 suspected or confirmed cases of microcephaly.

Zika has also been linked to Guillain-Barre syndrome, an immune system disorder that can occasionally lead to a fatal form of paralysis.

Speaking earlier this month, CDC director Dr. Tom Frieden said that "we are learning more about Zika every day. The link with microcephaly and other possibly serious birth defects is growing stronger every day. The link to Guillain-Barre syndrome is likely to be proven in the near future, and the documentation that sexual transmission is possible is now proven."

First discovered in Uganda in 1947, the Zika virus wasn't thought to pose major health risks until last year, when it became clear that it posed potentially devastating threats to pregnant women.

Meanwhile, the virus continues to spread in Latin America and the Caribbean.

It is not expected to pose a significant threat to the U.S. mainland, federal health officials have said in the past.

In Puerto Rico, however, the situation is "of great concern," Frieden said.

"Puerto Rico is on the frontline of the battle against Zika," said Frieden, who had just returned from the island. "And it's an uphill battle."

By next year, Frieden said, there could be hundreds of thousands of cases of Zika in the territory, and "thousands of infected pregnant women."

In a separate report released Friday, the CDC stressed that effective contraception needs to be made much more readily available to Puerto Ricans. In a statement, the agency noted that, "approximately two-thirds of pregnancies in Puerto Rico are unintended, indicating a potentially unmet need for access to birth control."

The agency said that the U.S. Department of Health and Human Services will boost its efforts at family planning education in Puerto Rico, so that women can help prevent unintended pregnancies -- especially those jeopardized by Zika infection.

The Zika virus has now spread to over 38 countries and territories, most in Latin America and the Caribbean. The World Health Organization estimates there could be up to 4 million cases of Zika in the Americas in the next year.